The main-transition temperatures between gel and liquid crystalline phases for dipalmitoylphosphatidylmonomethylethanolamine (DPMePE) and dipalmitoyl-phosphatidyldimethylethanolamine (DPMe_2PE) under high pressure were determined from differential scanning calorimetry (DSC) and optical methods. The thermodynamic quantities of main transition (enthalpy, entropy, and volume) were calculated and the results were compared with those of dipalmitoylphosphatidyl-ethanolamine (DPPE) and dipalmitoylphosphatidylcholine (DPPC) to clarify the effect of N-methylation of DPPE on the main transition. The main-transition temperatures of N-methylated DPPEs increased linearly by applying pressure while they decreased with increasing the size of head group by stepwise N-methylation. On the other hand, there was no significant difference in thermodynamic quantities between the phospholipids. The difference in hydrophilic interactions between the different-sized polar head groups produces a great difference on the main-transition temperature and the packing arrangement of bilayer membrane in the gel phase.
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